Press one of the eight track numbers located underneath the mixer's faders to select the track to record to. The buttons will be yellow when there are no previous recordings put onto them.
Press the "V-Track" button that is located to the left of the mixer panel on the recorder. This will tell the Boss BR-8 to record a virtual track.
Press "Record" and then "Play to begin recording a take to one of the virtual tracks. When you are satisfied with the recording, press "Stop." Repeat steps 1 through 3 for up to eight virtual tracks in total.